This invention is directed to an "improvement" in the art of acid zinc chloride plating baths. The zinc chloride electrolytes have been replacing the old alkaline zinc cyanide electrolytes in order to eliminate the toxic effluents from the cyanide baths. An additional advantage is that the zinc chloride baths will plate over carbonitrided steel parts that are difficult to impossible to plate in an alkaline zinc bath.
The pioneer bright zinc chloride electroplating baths were those of Korpium and Steeg disclosed in U.S. Pat. No. 3,694,330 of Sept. 26, 1972. These inventors utilized an electrolyte of zinc and ammonium chlorides, along with a complexing agent selected from the group consisting of glycero-phosphates, triethanolamine, ethylene diamine, and ethylene diamine tetracetic acid. For a brightening system; they utilized an aromatic carbonyl compound dispersed by means of an ethoxylated surfactant selected from the ethylene oxide condensation products with long chain fatty alcohols, fatty acids, or fatty amines, or with long chain alkyl phenols.
Variations of this original bright zinc chloride bath were made by utilizing other brightening agents as substitutes for the aromatic carbonyl compound, but continuing the use of the polyoxyethylated wetting agents. For example Rosenberg in U.S. Pat. No. 3,767,540 used polyimines with a polyoxyethylated napthol wetter, and Creutz in U.S. Pat. No. 3,909,373 used ethoxylated acetylenic glycol wetters along with isopropyl nicotinate. A departure from the use of the polyoxethylated wetting agents alone came from Beckwith and Hsu in U.S. Pat. No. 3,787,297 who introducted the use of a fatty sarcosine in conjunction with a fatty imidazoline and an ethoxylated carbamine sulfonate to disperse the aromatic carbonyl brighteners. The ethoxylated carbamine sulfonate wetter is used at twice the concentration of the other wetting agents in this system.